Bellcore has developed a series of requirements for telecommunications equipment manufacturers to follow when interfacing a digital loop carrier with a class V central office switch. These requirements are collected in Bellcore document TR-303.
One of the optional requirements of TR-303 concerns assembly of a plurality of Integrated Services Digital Network (ISDN) D-channel signals into a single DS0 signal. This option of TR-303 now requires that up to four ISDN D-channel signals be multiplexed together to form a single DS0 signal for transport from a digital loop carrier to a class V central office switch. The regional bell operating companies now require that this option be made available.
An ISDN signal comprises two B-channel and one D-channel signal. Each B-channel signal comprises an 8-bit signal while the D-channel signal comprises a 2-bit signal. Because the 8-bit DS0 signal has been the traditional base unit of transport for the telephone system, ISDN D-channel signals often consume an entire 8-bit DS0 signal. Transporting individual D-channel signals in a DS0 signal wastes three-fourths of the bandwidth for that particular DS0.
The optional requirement of TR-303 discussed above seeks to utilize bandwidth more efficiently. In accordance with TR-303, one to four ISDN D-channel signals may be multiplexed together and carried in a single DS0 signal between the digital loop carrier and the central office class V switch. This TR-303 requirement reduces wasted bandwidth between a digital loop carrier and a class V central office switch, particularly where a large number of ISDN lines are connected to the digital loop carrier.